1. Field of the Invention
The present invention relates to a process and a device for controlling the speed of an internal combustion engine during a deceleration phase, and more specifically to a process and a device of this type which operate by correcting the control of an actuator which acts on this speed as a function of the deviation between a set-point speed and the actual speed.
Internal combustion engines, particularly those which propel automobiles, run at variable speeds, the control and/or adjustment of which is often delicate, particularly during the deceleration phase. A deceleration phase usually begins when the driver lifts his foot off the accelerator. The object of speed control during such a phase is to assure the return of this speed to a set-point speed, the adjustment of the speed around this set-point speed despite potential disturbances, and the passage through various transitory phases such as a "driven" deceleration phase in which the vehicle runs with an engaged gear box ratio, or an engine start-up phase.
In all of these circumstances, control of the speed is quite delicate, since it is known that the stability of an engine at low speed is difficult to assure and that the reactions of the engine are difficult to model. Moreover, the conditions for the onset of a deceleration phase can vary considerably, for example in relation to the driver's action on the accelerator pedal, the engine coolant temperature, the air temperature, and the potential presence of random disturbances due to the engagement of an electrical (lighting device, ventilator) or mechanical (air conditioner, power steering) device. The speed control must also take into account other constraints associated with the driver's comfort (noise level, vibrations, jerking) and to standards related to the pollution of the environment by the exhaust gases from the engine.
2. Description of the Related Art
At present, in order to assure control of the speed of the engine during a deceleration phase, closed loop control devices with "supervised" PID-type controllers are commonly used. A device of this type is described in German patent Disclosure DE-A-4 215 959, for example, which relies on fuzzy logic to adjust the P, I and D terms of the controller. The result is time-consuming, tedious tuning of the controller in order to adapt it to each type of engine. The PID adjustment is also disadvantageous in that it takes into account only certain aspects of the operation of the engine, and that it is not entirely satisfactory from the standpoint of "robustness", since engine aging or manufacturing tolerances for engines can unfavorably affect the operation of a "supervised" PID controller.
A process for controlling the deceleration speed of an internal combustion engine, which is based entirely on experimental results implemented with the aid of fuzzy logic and is therefore likely, a priori, to have greater robustness and flexibility, is known from SAE document No. 900594, published by the Society of Automotive Engineers of the United States of America. However, the process described requires the utilization of tables and complex operators which take up a lot of space in the memory of the computer used to implement the process, which moreover involves long calculation times.